The present invention relates to a method and an apparatus for producing an oil reservoir. The invention is developed in connection with the demand and need for production of deeper localized and mostly relatively small oil reservoirs. When producing such oil reservoirs it is regularly necessary to inject water in the resevoir thereby forcing out the hydrocarbons from the reservoir.
Increasingly, oil producers are looking for ways to produce oil from wells in which the production fluid has a high water cut efficiently enough to make the cost of production economic. One known method is to separate the production fluid in a downhole separator comprising one or a series of hydrocyclones in order to provide initial separation of at least some of the water from the production fluid to reduce the quantity of production fluid which needs to be transported to the surface. The separated water is re-injected, to the oil reservoir or another site.
According to the present invention a method for producing an oil reservoir is provided,
comprising establishing an oil producing well,
producing a production fluid containing oil and water,
transporting the production fluid to a downhole liquid/liquid hydrocyclone,
separating the production fluid in the liquid/liquid hydrocyclone into an oil enriched stream at the hydrocyclone overflow, and a water enriched stream at the hydrocyclone underflow,
transporting the oil enriched stream to the surface,
re-injecting the water enriched stream at a downhole site, and
cooling the transported oil enriched stream by sending a counterflow of a cooling medium relative the oil enriched stream, thereby keeping the oil enriched stream in a substantially liquid phase.
By cooling the ascending oil enriched stream the stream liquid phase is kept as long as possible, thereby substantially avoiding gas formation.
When producing a reservoir containing volatile oils the production fluid will be subject to a volatilazation caused by a pressure reduction upwards in the well. A cooling of the upwards flowing upstream in the well will cause a temperature reduction balancing the pressure reduction, whereby the condensation curve in the corresponding phase diagram is not crossed. Thus gas will not evolve and a multi-phase situation is avoided. The result is a preventive liquidation in that the stream is hindered in giving away the volatile heat and will remain in the same phase, that is the liquid phase. Some of the most volatile components will of course evolve and remain in the gaseous phase bit it is estimated that the bulk of the gas phase will not evolve but remain in the liquid.
The counterflow may advantageously be used as the driving medium for a pump transporting the oil enriched stream.
The cooling medium may preferably include injection water, delivered from a water pump at or above the surface.
Provided the cooling is not sufficient to avoid a disturbing gas phase the oil enriched stream can be pressurized in one or more additional downhole pumps positioned in the well above the transportation pump. Thereafter the stream is cooled further, striving to keep the stream outside the condensation line in the phase diagram.
The cooling medium may include re-injection water provided from an aquifier, i.e. an underground site containing water. Water drawn from an aquifier is preferably separated in a downhole solid/liquid hydrocyclone, whereby the separated water is re-injected in the producing reservoir and the solids are re-injected in the ground, for instance in the aquifier.
The present invention also provides an apparatus for producing an oil reservoir, the apparatus comprising a downhole liquid/liquid hvdrocyclone for separating production fluid containing oil and water into an oil enriched stream and a water enriched stream, a downhole pump for pumping the oil enriched stream to the surface, and means for sending a counterflow of cooling medium relative the oil enriched stream.
The means for sending the counterflow may preferably include a means for sending a flow of injection water down the well.
Preferably the downhole transporting pump may be in driven connection with a downhole injection water driven turbine.
Also a re-injection water pump may preferably be in driven connection with said downhole turbine.
As disclosed above in connection with the inventive method the apparatus will preferably include one or more additional downhole pumps for transport (pressurizing) of the cooled oil enriched stream.
The apparatus may further include a downhole solid/liquid hvdrocyclone in the well at a position above the downhole transport pump, said hydrocyclone being flow coupled to an aquifier by means of a pump and having an overflow delivering separated water down to the oil producing reservoir, and an underflow delivering separated solids to an underground site.